Anaerobic digestion at mesophilic and thermophilic temperature

During anaerobic digestion of organic wastes, nutrient-rich digestate is produced. This digestate has great potential as a fertiliser on farmland. However, one concern is the content of organic pollutants in the digestate, as these may influence the soil fertility in the long-term perspective. Different organic pollutants have been found in various organic wastes and types of digestate. This thesis describes the effect of temperature on the anaerobic degradation of different organic pollutant i.e. aromatic compounds and on the structure of microbial communities. To isolate the impact of temperature on the degradation, investigations were carried out in two laboratory-scale bioreactors that were stable and equivalent in performance apart from process temperature and hydraulic retention time. These reactors have treated the same type of organic municipal household waste for many years. The aromatic compounds were chosen to represent important anaerobic reaction steps and key intermediates. Several of the selected compounds were efficiently degraded at the mesophilic temperature (37 ºC). In contrast, phenols and phthalic acid were not degraded at the thermophilic temperature (55 ºC). This limited degradation at the higher temperature is contrary to the general degradation rate of organic matter, which increase with elevated temperatures. Chemical analysis of the digestate from the laboratory-scale reactor operating at thermophilic temperature detected a comparably higher content of phenols, verifying the degradation results. The effect of temperature on phenol degradation was further confirmed in a study including several large-scale bioreactors. To investigate the impact of temperature on microbial community structure, phylogenetic analysis was performed with 16S rRNA genes from the archaeal and bacterial communities in the same two laboratory-scale bioreactors as used for the degradation study. This analysis revealed a lower microbial diversity at the higher temperature. This difference in diversity can possibly explain the observed difference in degradation capacity. The microorganisms responsible for the degradation of phenol were studied in enrichment cultures originating from the two laboratory-scale bioreactors. Two different bacteria were suggested to perform the phenol degradation in these cultures, a finding that highlights the influence of temperature on community structure